[X4U] Wierd Freeze; External HD??

[net kat]

I have a 1.42Ghz Mac Mini wtih 1GB of apple-installed RAM and in the  
past 2 days, I've experienced a wierd freeze..

what has happened is that I have had some 3D rendering going on (the  
kind that takes a day or days, 24-7. processor but not disc intensive).
the first time, I heard my external 7200RPM drive, also my boot  
volume, make a normal hard drive sound, similar to parking or  
sleeping, yet it didn't sleep (that I know of).

Next thing I know, all apps and the entire OS become unreceptive.  
I'll use CMD-Tab to see if I can get to the finder, and that's where  
things freeze totally.

If I shut everything down, then restart turning external drive on  
first, then Mini, I can get back to where I was and resume.

I do not  have any energy saver things set on, and have 10.4.4

Any ideas on whassup?

thanks!

nk